Aqueous alkanolamine composition and process for the removal of acid gases from gaseous mixtures

ABSTRACT

The present invention relates to an aqueous alkanolamine solution for the removal of hydrogen sulfide from gaseous mixtures. The aqueous alkanolamine solution comprises (i) an amino compound with the formula: R 1 R 2 NCH 2 CH(OH)CH 2 OH wherein R 1  and R 2  independently represent lower alkyl groups of 1 to 3 carbon atoms and (ii) an acid or acid-forming material having a pKa equal to or less than 8, optionally (iii) another amino compound, and optionally (iv) a physical solvent. Further, the present invention relates to a process for removing acid gases from a gaseous mixture, preferably hydrogen sulfide, comprising the step of contacting the gaseous mixture with the aqueous alkanolamine solution, preferably wherein the temperature of the aqueous alkanolamine solution is equal to or greater than 140° F. Examples of the gaseous mixtures include natural gas, synthesis gas, tail gas, and refinery gas.

FIELD OF THE INVENTION

The present invention relates to a composition comprising an aqueoussolution of an acid or an acid-forming material and an alkanolamine,preferably 3-(dimethylamino)-1,2-propanediol, and a process for usingsaid aqueous composition for removing acid gases, such as CO₂, COS, andH₂S, from gaseous mixtures, preferably, removing H₂S from gaseousmixtures under conditions wherein the aqueous solution is at an elevatedtemperature.

BACKGROUND OF THE INVENTION

Fluid streams derived from natural gas reservoirs, petroleum or coal,often contain a significant amount of acid gases, for example carbondioxide (CO₂), hydrogen sulfide (H₂S), sulfur dioxide (SO₂), carbondisulfide (CS₂), hydrogen cyanide (HCN), carbonyl sulfide (COS), ormercaptans as impurities. Said fluid streams may be gas, liquid, ormixtures thereof, for example gases such as natural gas, refinery gas,hydrocarbon gasses from shale pyrolysis, synthesis gas, and the like orliquids such as liquefied petroleum gas (LPG) and natural gas liquids(NGL).

Various compositions and processes for removal of acid gasses are knownand described in the literature. It is well-known to treat gaseousmixtures with aqueous amine solutions to remove these acidic gases.Typically, the aqueous amine solution contacts the gaseous mixturecomprising the acidic gases counter currently at low temperature andhigh pressure in an absorber tower. The aqueous amine solution commonlycontains an alkanolamine such as triethanolamine (TEA),methyldiethanolamine (MDEA), diethanolamine (DEA), monoethanolamine(MEA), diisopropanolamine (DIPA), or 2-(2-aminoethoxy)ethanol (sometimesreferred to as diglycolamine or DGA). In some cases, an accelerator, isused in combination with the alkanolamines, for example piperazine andMDEA as disclosed in U.S. Pat. Nos. 4,336,233; 4,997,630; and 6,337,059,all of which are incorporated by reference herein in their entirety.Alternatively, EP 0134948 discloses mixing an acid with select alkalinematerials such as MDEA, to provide enhanced acid gas removal. However,EP0134948 teaches that only a select class of alkaline materials mixedwith an acid are useable in aqueous alkaline solutions to provideincreased acid gas removal.

Tertiary amines, such as 3-dimethylamino-1,2-propanediol (DMAPD), havebeen shown to be effective at removing CO₂ from gaseous mixtures, seeU.S. Pat. No. 5,736,116. Further, in specific processes, e.g., theGirbotol Process, tertiary amines have been shown effective in removalof H₂S, but show decreased capacity at elevated temperatures, forexamples see “Organic Amines-Girbotol Process”, Bottoms, R. R., TheScience of Petroleum, volume 3, Oxford University Press, 1938, pp1810-1815.

While the above compounds are effective, they each have limitationswhich detract from their universal use. In particular, it would bedesirable to have and aqueous composition comprising an alkanolamine forremoving H₂S from a gaseous mixture and/or an aqueous alkanolaminesolution which is efficient at removing acid gases at a commerciallyviable capacity when the aqueous solution is used at an elevatedtemperature, for example above 140° F.

As such, there is a need for an aqueous alkanolamine solution and methodto use said solution, which is effective at removing hydrogen sulfidefrom gaseous mixtures and/or removing acid gases at elevated operatingtemperatures.

SUMMARY OF THE INVENTION

The present invention is an aqueous alkanolamine solution compositionand process using said aqueous alkanolamine solution composition forremoving acid gases, preferably removing hydrogen sulfide throughcontact with gaseous mixtures comprising said acid gases, preferablywherein the temperature of the aqueous alkanolamine solution is equal toor greater than 140° F., said composition comprising (i) an aminocompound, preferably in an amount of from 0.1 to 75 weight percent,having the general formula:

R¹R²NCH₂CH(OH)CH₂OH   (1)

wherein R¹ and R² independently represent lower alkyl groups of 1 to 3carbon atoms, for example, methyl, ethyl, propyl, and isopropyl groups,more preferred R¹ and R² groups include methyl and ethyl groups,especially preferred amino compounds include3-dimethylamino)-1,2-propanediol in which R¹ and R² are both methylgroups, and 3-diethylamino)-1,2-propanediol in which R¹ and R² are bothethyl groups; (ii) an acid or an acid-forming material, preferably in anamount of from 0.1 to 25 weight percent, such as an organic or inorganicacid having a pKa of 8 or less, preferably 7 or less, more preferably 6or less; (iii) one or more additional amino compound, preferably in anamount of from 0 to 75 weight percent, which is different from the aminocompound described herein above (i), preferred additional aminocompounds comprise one or more tertiary amino group, and (iv) optinallya physical solvent, preferably selected from cyclotetramethylenesulfone,dimethyl ethers of polyethylene glycol,1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone, N-formylmorpholine,N-acetylmorpholine, triethylene glycol monomethyl ether, or mixturesthereof.

wherein weight percents are based on the total weight of the aqueousalkanolamine solution.

In one embodiment of the present invention, the amino compound (i)preferably is 3-(dimethylamino)-1,2-propanediol or3-(diethylamino)-1,2-propanediol.

In one embodiment of the present invention, preferably the acid (ii) isphosphoric acid, sulfuric acid, boric acid, formic acid, or hydrochloricacid.

In one embodiment of the present invention, the process furthercomprises the step of steam stripping the aqueous alkanolamine solutionsuch that a hydrogen sulfide-lean aqueous alkanolamine solution isformed which may be used in said contacting step.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a process flow diagram of an absorption processaccording the present invention.

FIG. 2 is a plot of H₂S concentration in a cleaned gas mixture versusthe absorbent circulation rate.

DETAILED DESCRIPTION OF THE INVENTION

The aqueous solvent of the present invention is an aqueous aminesolution comprising an alkanolamine compound and an acid or anacid-forming material. The amino compounds useful in the aqueousalkanolamine solutions of the present invention have the generalformula:

R¹R²NCH₂CH(OH)CH₂OH   (1)

wherein R¹ and R² independently represent lower alkyl groups of 1 to 3carbon atoms, for example, methyl, ethyl, propyl, and isopropyl groups.More preferred R¹ and R² groups include methyl and ethyl groups.Especially preferred amino compounds include3-(dimethylamino)-1,2-propanediol in which R¹ and R² are both methylgroups, and 3-(diethylamino)-1,2-propanediol in which R¹ and R² are bothethyl groups.

The aqueous alkanolamine solution of the present invention contains thealkanolamine in an amount equal to or greater than 0.1 weight percent,preferably equal to or greater than 5 weight percent, more preferablyequal to or greater than 10 weight percent and even more preferablyequal to or greater than 20 weight percent wherein weight percent isbased on the total weight of the aqueous solution. The aqueousalkanolamine solution of the present invention contains the alkanolaminein an amount equal to or less than 75 weight percent, preferably equalto or less than 65 weight percent, more preferably equal to or less than55 weight percent and even more preferably equal to or less than 50weight percent wherein weight percent is based on the total weight ofthe aqueous solution.

Suitable acids or acid-forming materials that can be used in the presentinvention can be characterized as strong acids which include any organicor inorganic acid having a pKa of 8 or less, preferably 7 or less, morepreferably 6 or less. Acids that can be used include phosphoric acidwhich is preferred because of its low corrosive effects, phosphorusacid, boric acid, hydrochloric acid, sulfuric acid, boric acid,sulfurous acid, nitrous acid, pyrophosphoric acid, telurous acid, andthe like. Also included as suitable acids are organic acids such asacetic acid, formic acid, adipic acid, benzoic acid, n-butyric acid,chloroacetic acid, citric acid, glutaric acid, lactic acid, malonicacid, oxalic acid, o-phthalic acid, succinic acid, o-toluic acid, andthe like. In addition, acid-forming materials that are capable offorming acids upon contact with water can be used. The acids formed fromsuch acid-forming materials useful in the present invention have a pKaof 8 or less, preferably 7 or less, and more preferably 6 or less. Asuitable acid-forming material is sulfur dioxide.

The aqueous alkanolamine solution of the present invention contains theacid and/or acid-forming material in an amount equal to or greater than0.1 weight percent, preferably equal to or greater than 0.5 weightpercent, more preferably equal to or greater than 1 weight percentwherein weight percent is based on the total weight of the aqueoussolution. The aqueous alkanolamine solution of the present inventioncontains the acid or the acid-forming material in an amount equal to orless than 25 weight percent, preferably equal to or less than 10 weightpercent, more preferably equal to or less than 5 weight percent and evenmore preferably equal to or less than 2.5 weight percent wherein weightpercent is based on the total weight of the aqueous solution.

The aqueous absorbent composition of the present invention mayoptionally contain one or more additional amino compound. Preferably,the additional amino compound is a different or second alkanolamine notdescribed by formula (1) herein above, such as tris(2-hydroxyethyl)amine(triethanolamine, TEA); tris(2-hydroxypropyl)amine (triisopropanol);tributanolamine; bis(2-hydroxyethyl)methylamine (methyldiethanolamine,MDEA); 2-diethylaminoethanol (diethylethanolamine, DEEA);2-dimethylaminoethanol (dimethylethanolamine, DMEA);3-dimethylamino-1-propanol; 3-diethylamino-1-propanol;2-diisopropylaminoethanol (DIEA); N,N-bis(2-hydroxypropyl)methylamine(methyldiisopropanolamine, MDIPA); N,N′-bis(2-hydroxyethyl)piperazine(dihydroxyethylpiperazine, DiHEP)); diethanolamine (DEA);2-(tert-butylamino)ethanol;

2-(tert-butylaminoethoxy)ethanol; or 2-amino-2-methylpropanol (AMP),2-(2-amino-ethoxy)ethanol.

Preferred additional amino compounds comprise one or more tertiary aminogroup.

Preferably the additional amino compound has one or more stericallyhindered amino group. An aqueous absorption composition comprising a1-hydroxyethyl-4-pyridnlypiperazine compound and an amine having one ormore sterically hindered amino group is particularly suitable for theremoval of H₂S.

If present, the amount of optional amino compound in the aqueousalkanolamine solution may range from equal to or greater than 0.1 weightpercent, preferably equal to or greater than 1 weight percent, morepreferably equal to or greater than 5 weight percent based the totalweight of the aqueous alkanolamine solution. If present, the amount ofoptional amino compound in aqueous alkanolamine solution may range fromequal to or less than 75 weight percent, preferably equal to or lessthan 50 weight percent, more preferably equal to or less than 25 weightpercent based the total weight of the aqueous alkanolamine solution.

For removing H₂S from a gaseous mixture, the temperature of the aqueousalkanolamine solution which is brought into contact with the gas to betreated is equal to or greater than 120° F., preferably equal to orgreater than 130° F., more preferably equal to or greater than 140° F.,and even more preferably equal to or greater than 150° F.

In addition to the amino compound and acid, the aqueous alkanolaminesolution may comprise one or more other compounds used in fluidtreatment following well known practices. Illustrative compounds whichmay optionally be provided include, but are not limited to, one or moreof the following: antifoaming agents; physical solvents includingglycols and the mono-and di-ethers or esters thereof, aliphatic acidamides, N-alkylated pyrrolidones, sulfones, sulfoxides and the like;antioxidants; corrosion inhibitors; film formers; chelating agents suchas metals; pH adjusters such as alkali compounds; and the like. Theamount of these optional components is not critical but may be providedin an effective amount following known practices.

In addition to the amino compound, the acid, and the optional one ormore other compounds used in fluid treatment the aqueous alkanolaminesolution may comprise a physical solvent. Preferably a solvent such ascyclotetramethylenesulfone (available under the tradename SULFOLANE,dimethyl ethers of polyethylene glycol (available under the tradenameSELEXOL from The Dow Chemical Company), and triethylene glycolmonomethyl ether (TGME or METHOXYTRIGLYCOL from The Dow ChemicalCompany), 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone,N-formylmorpholine, N-acetylmorpholine, or mixtures thereof.

If present, the amount of physical solvent in the aqueous alkanolaminesolution may be present in an amount from equal to or greater than 1weight percent, preferably equal to or greater than 5 weight percent,more preferably equal to or greater than 10 weight percent based thetotal weight of the aqueous alkanolamine solution. If present, theamount of physical solvent in the aqueous alkanolamine solution may bepresent in an amount equal to or less than 75 weight percent, preferablyequal to or less than 65 weight percent, more preferably equal to orless than 50 weight percent based the total weight of the solution.

The invention set forth herein has great application in thepetrochemical and energy industries. For example, the present inventioncan be used for the treatment of fluid streams, gas, liquid, ormixtures, in an oil refinery, the treatment of sour gas, the treatmentof coal steam gas, the treatment of hazardous stack emissions, thetreatment of land field gasses, and a new series of devices dealing withhazardous emissions for human safety.

The fluid streams to be treated by the process of the present inventioncontain an acid gas mixture which includes H₂S, and may optionallyinclude other gases such as CO₂, N₂, CH₄, C₂H₆, C₃H₈, H₂O, CO, H₂O, COS,HCN, NH₃, O₂, mercaptans, and the like. Often such gas mixtures arefound in combustion gases, refinery gases, town gas, natural gas, syngas, tail gas, water gas, propane, propylene, heavy hydrocarbon gases,etc. The aqueous alkanolamine solution herein is particularly effectivewhen the fluid stream is a gaseous mixture, obtained, for example, fromshale oil retort gas, coal or gasification of heavy oil with air/steamor oxygen/steam thermal conversion of heavy residual oil to lowermolecular weight liquids and gases, or in sulfur plant tail gas clean-upoperations.

The process of the present invention is preferably used to remove H₂Sfrom a gas stream comprising H₂S optionally in the presence of one ormore other acid gas impurities, for example CO₂, N₂, CH₄, C₂H₆, C₃H₈,H₂, CO, H₂O, COS, HCN, NH₃, O₂, and/or mercaptans. However, the presentinvention may be used to remove H₂S and one or more of CO₂, N₂, CH₄,C₂H₆, C₃H₈, H₂, CO, H₂O, COS, HCN, NH₃, O₂, and/or mercaptans from a gasstream comprising H₂S and one or more of CO₂, SO₂, CS₂, HCN, COS, and/ormercaptans.

The absorption step of this invention generally involves contacting thefluid stream, preferably gaseous mixture, with the aqueous alkanolaminesolution in any suitable contacting vessel, for examples ofrepresentative absorption processes see U.S. Pat. Nos. 5,736,115 and6,337,059, both of which are incorporated herein by reference in theirentirety. In such processes, the fluid stream containing H₂S andoptionally CO₂ and/or other impurities from which the H₂S is to beremoved may be brought into intimate contact with the aqueousalkanolamine solution using conventional means, such as a tower orvessel packed with, for example, rings or with sieve plates, or a bubblereactor.

In a typical mode of practicing the invention, the absorption step isconducted by feeding the fluid stream into the lower portion of theabsorption tower while fresh aqueous alkanolamine solution is fed intothe upper region of the tower. The fluid stream, freed largely from theH₂S, emerges from the upper portion (sometimes referred to as treated orcleaned gas) of the tower, and the loaded aqueous alkanolamine solution,which contains the absorbed H₂S, leaves the tower near or at its bottom.Preferably, the inlet temperature of the absorbent composition duringthe absorption step is in the range of from 120° F. to 210° F., and morepreferably from 140° F. to 200° F. Pressures may vary widely; acceptablepressures are between 5 and 2,000 pounds per square inch (psi),preferably 20 to 1,500 psi, and most preferably 25 to 1,000 psi in theabsorber. The contacting takes place under conditions such that the H₂Sis preferably absorbed by the solution. The absorption conditions andapparatus are designed so as to minimize the residence time of theaqueous alkanolamine solution in the absorber to reduce CO₂ pickup whileat the same time maintaining sufficient residence time of the fluidstream with the aqueous absorbent composition to absorb a maximum amountof the H₂S gas. Fluid streams with low partial pressures, such as thoseencountered in thermal conversion processes, will require less of theaqueous alkanolamine solution under the same absorption conditions thanfluid streams with higher partial pressures such as shale oil retortgases.

A typical procedure for the H₂S removal phase of the process comprisesabsorbing H₂S via countercurrent contact of a gaseous mixture containingH₂S and CO₂ with the aqueous alkanolamine solution of the amino compoundin a column containing a plurality of trays at a temperature, of atleast 120° F., and at a gas velocity of at least 0.3 feet per second(ft/sec, based on “active” or aerated tray surface), depending on theoperating pressure of the gas, said tray column having fewer than 20contacting trays, with, e.g., 4 to 16 trays being typically employed.

After contacting the fluid stream with the aqueous alkanolaminesolution, which becomes saturated or partially saturated with H₂S, thesolution may be at least partially regenerated so that it may berecycled back to the absorber. As with absorption, the regeneration maytake place in a single liquid phase. Regeneration or desorption of theacid gases from the aqueous alkanolamine solution may be accomplished byconventional means of heating, expansion, stripping with an inert fluid,or combinations thereof, for example pressure reduction of the solutionor increase of temperature to a point at which the absorbed H₂S flashesoff, or by passing the solution into a vessel of similar construction tothat used in the absorption step, at the upper portion of the vessel,and passing an inert gas such as air or nitrogen or preferably steamupwardly through the vessel. The temperature of the solution during theregeneration step should be in the range from 120° F. to 210° C., andpreferably from 140° F. to 200° F., and the pressure of the solution onregeneration should range from 0.5 psi to 100 psi, preferably 1 psi to50 psi. The aqueous alkanolamine solution, after being cleansed of atleast a portion of the H₂S gas, may be recycled back to the absorbingvessel. Makeup absorbent may be added as needed.

In a preferred regeneration technique, the H₂S-rich aqueous alkanolaminesolution is sent to the regenerator wherein the absorbed components arestripped by the steam which is generated by boiling the solution.Pressure in the flash drum and stripper is usually 1 psi to 50 psi,preferably 15 psi to 30 psi, and the temperature is typically in therange from 120° F. to 340° F., preferably 170° F. to 250° F. Stripperand flash temperatures will, of course, depend on stripper pressure;thus at 15 psi to 30 psi stripper pressures, the temperature will be170° F. to 250° F. during desorption. Heating of the solution to beregenerated may very suitably be affected by means of indirect heatingwith low-pressure steam. It is also possible, however, to use directinjection of steam. The resulting hydrogen sulfide-lean aqueousalkanolamine solution may be used to contact a gaseous mixturecontaining H₂S.

Preferably the clean gas contains equal to or less than 10 ppm H₂S,meeting some environmental regulations, more preferably equal to or lessthan 4 ppm H₂S, meeting typical pipeline specifications.

A preferred embodiment of the present invention involves performing themethod of the present invention continuously, or as a continuousprocess. However, the method may be performed batch wise orsemi-continuously. Selection of the type of process used should bedetermined by the conditions, equipment used, type and amount of gaseousstream, and other factors apparent to one of ordinary skill in the artbased on the disclosure herein.

EXAMPLES

Examples 1 to 12 are an aqueous amine absorbent solution comprising 50parts by weight of an alkanolamine, 50 parts by weight deionized water,and optionally 1 or 2 parts by weight of an acid, parts by weight arebased on the total weight of the aqueous amine absorbent solution. A gasstream comprising a synthetic mixture containing 4.2 percent H₂S, 16percent CO₂ and 79.8 percent N₂, wherein percent is percent by volume,is treated in a pilot scale absorber to remove the H₂S and CO₂. For eachaqueous amine absorbent solution, the gas stream is treated at threedifferent flow rates. The compositions, process parameters, and residualH₂S and CO₂ levels for Examples 1 to 12 are listed in Table 1. In Table1:

“MDEA” is 98% methyldiethanolamine available from The Dow ChemicalCompany;

“DMAPD” is 98% 3-dimethylamino-1,2-propanediol available from AKscientific; and

“H₃PO₄” is an 85% o-phosphoric acid available from Fisher Scientific.

An aqueous amine absorbent solution is introduced into the pilot scaleabsorber FIG. 1 via feed line 5 into the upper portion of a gas-liquidcountercurrent packed-bed absorption column 2. The gas stream isintroduced through feed line 1 into the lower portion of column 2 at agas flow rate of 10 liter per minute. The absorber pressure is adjustedto 238 psia. The clean gas (i.e., reduced amounts of H₂S and CO₂) isdischarged at the top of the absorber 2 through line 3 and residual H₂Sand CO₂ levels are determined by gas chromatography (GC) analysis. Theaqueous amine solution loaded with H₂S and CO₂ flows toward the lowerportion of the absorber, and leaves via line 4.

The aqueous amine in line 4 is reduced in pressure by the level controlvalve 8 and flows through line 7 to heat exchanger 9, which heats theloaded aqueous solution. The hot rich solution enters the upper portionof the regenerator 12 via line 10. The regenerator 12 is equipped withrandom packing which effects desorption of the H₂S and CO₂ gases. Thepressure of the regenerator is set at 17 psia. The gases are passedthrough line 13 into condenser 14 wherein cooling and condensation ofany residual water and amine occurs. The gases enter a separator 15wherein the condensed liquid is separated from the vapor phase. Thecondensed aqueous solution is pumped via pump 22 through line 16 to theupper portion of the regenerator 12. The gases remaining from thecondensation are removed through line 17 for final collection and/ordisposal. The regenerated aqueous solution flows down through theregenerator 12 and the close-coupled reboiler 18. The reboiler 18,equipped with an electrical heating device, vaporizes a portion of theaqueous solution to drive off any residual gases. The vapors rise fromthe reboiler and are returned to the regenerator 12 which comingle withfalling liquid and then exit through line 13 for entry into thecondensation stage of the process. The regenerated aqueous solution fromthe reboiler 18 leaves through line 19 and is cooled in heat exchanger20, and then is pumped via pump 21 back into absorber 2 through feedline 5.

The flow rate for the aqueous amine absorbent is determined by slowlyadjusting downward until the amount of H₂S in the purified gas line 3shows a dramatic increase.

The results for Examples 1 to 12 are graphically represented in the plotshown in FIG. 2. H₂S levels, in parts per million by volume (ppmv), areplotted against the amine flow rate in cubic centimeters per minute(cc/min).

TABLE 1 Example 1* 2* 3* 4* 5* 6* 7 8 9 10 11 12 Absorbent CompositionMDEA 50 50 50 DMAPD 50 50 50 50 50 50 50 50 50 H₃PO₄ 1 1 1 1 1 1 2 2 2Water 50 50 50 50 50 50 50 50 50 50 50 50 Absorbent Flow Rate, cc/min36.4 24.9 21.4 26.7 22.5 19.7 24.9 22.3 17.5 24.6 20.2 17.1 Outlet GasGC Analysis CO₂, ppmv 4.6 8 9.2 5.1 7.5 7.5 7.2 8.5 9.8 6 8 9.1 H₂S,ppmv 7.6 71 370 44 99 228 21.4 44 410 2.5 16 155 Lean SolutionTemperature, ° F. 152 152 152 150 150 150 151 151 151 150 150 150 InletGas Temperature, ° F. 128 128 128 129 129 129 128 128 128 129 129 129*Not an example of the present invention

What is claimed is:
 1. An aqueous alkanolamine solution for the removalof hydrogen sulfide from gas mixtures comprising (i) an amino compoundwith the formula:R¹R²NCH₂CH(OH)CH₂OH wherein R¹ and R² independently represent methyl,ethyl, propyl, or isopropyl groups, (ii) an acid having a pKa of 8 orless or an acid-forming material capable of forming in aqueous medium anacid having a pKa of 8 or less, and (iii) optionally one or more aminocompound different than (i).
 2. The aqueous alkanolamine solution ofclaim 1 wherein (i) the amino compound is present in an amount of from0.1 to 75 weight percent, (ii) the acid is present in an amount of from0.1 to 25 weight percent, and (iii) the optional amino compound ispresent in an amount of from 0 to 75 weight percent, wherein weightpercent is based on the total weight of the aqueous alkanolaminesolution.
 3. The aqueous alkanolamine solution of claim 1 wherein theamino compound (i) is 3-(dimethylamino)-1,2-propanediol or3-(diethylamino)-1,2-propanediol.
 4. The aqueous alkanolamine solutionof claim 1 wherein the acid is phosphoric acid, sulfuric acid, boricacid, formic acid, or hydrochloric acid.
 5. The aqueous alkanolaminesolution of claim 1 further comprises (iv) a physical solvent.
 6. Theaqueous alkanolamine solution of claim 5 wherein the physical solvent(iv) is selected from cyclotetramethylenesulfone, dimethyl ethers ofpolyethylene glycol, 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone,N-formylmorpholine, N-acetylmorpholine, triethylene glycol monomethylether, or mixtures thereof.
 7. A process for removing hydrogen sulfidefrom a gaseous mixture including hydrogen sulfide comprising the step ofcontacting the gaseous mixture with the aqueous alkanolamine solution ofany of the claims 1 to
 6. 8. The process of claim 5 wherein thetemperature of the aqueous alkanolamine solution is equal to or greaterthan 140° F.
 9. The process of claims 7 and 8 further comprising thestep of steam stripping the aqueous alkanolamine solution such that ahydrogen sulfide-lean aqueous alkanolamine solution is formed which maybe used in said contacting step.